The immune system uses many mechanisms for attacking pathogens; however, not all of these mechanisms are necessarily activated after immunization. Protective immunity induced by vaccination is dependent on the capacity of the vaccine to elicit the appropriate immune response to resist or eliminate the pathogen. Depending on the pathogen, this may require a cell-mediated and/or humoral immune response.
The current paradigm for the role of helper T cells in the immune response is that they can be separated into subsets on the basis of the cytokines they produce, and that the distinct cytokine profile observed in these cells determines their function. This T cell model includes two major subsets: TH-1 cells that produce IL-2 and interferon γ (IFN-γ) which augment both cellular and humoral immune responses, and TH-2 cells that produce IL-4, IL-5 and IL-10 which augment humoral immune responses (Mosmann et al., J. Immunol. 126:2348 (1986)). It is often desirable to enhance the immunogenic potency of an antigen in order to obtain a stronger immune response in the organism being immunized and to strengthen host resistance to the antigen-bearing agent. A substance that enhances the immunogenicity of an antigen with which it is administered is known as an adjuvant. For example, certain lymphokines have been shown to have adjuvant activity, thereby enhancing the immune response to an antigen (Nencioni et al., J. Immunol. 139:800-804 (1987); EP285441 to Howard et al.).